This invention relates generally to signal processing, and more particularly, to low pass filtering using a deadband.
Conventional low pass filters using resistors and capacitors work quite well in many, but not all, applications. In some instances, the inherent phase shift of the input signal causes a delay that is significant enough to cause problems.
For example, in many engine applications, a sensor detects the rotational speed or position of the engine and transmits a signal indicative thereof. FIG. 1 is a graph showing one example of an engine speed signal 10 transmitted from such a sensor. The engine was set to run at 2140 revolutions per minute (rpm) plus or minus 4 rpm. Thus, the portion of the engine speed signal 10 outside this range is noise, which may be filtered out to some extent by a conventional low pass filter.
FIG. 2 shows a graph of a modeled engine speed signal 12 of 2149 rpm plus or minus 3 rpm (generated by a frequency generator) transmitted from the engine speed sensor, along with a filtered version (an output signal 14) created by filtering the modeled engine speed signal 12 with a conventional (5 Hz) resistor/capacitor type low pass filter. As you can see, the modeled engine speed signal 12 contains portions having an amplitude outside of the 2149 plus or minus 3 rpm, which is noise. While the conventional low pass filter reduces the noise so that the amplitude of the output signal 14 is within the expected parameters (2149 plus or minus 3 rpm), the output signal 14 is delayed (phase shifted) by approximately 50 msec from the input signal.
In engine applications, it is not unusual for the conventional low pass filter to cause a phase shift of 100 msec. This is problematic in that, depending on the application, the engine speed signal may be read every 20 msec. Thus, the conventional low pass filter may process the input signal more slowly than is desired.
The present invention provides apparatuses and methods for filtering a signal. A first processing device receives a first control signal and a first feedback signal. The first processing device transmits a first error signal as a function of the first control signal and the first feedback signal. A second processing device is coupled with the first processing device to receive the first error signal. The second processing device transmits a second control signal as a function of the first error signal, the second control signal being substantially indicative of zero when the absolute value of the first error signal is less than or equal to a first predetermined value, substantially indicative of the first error signal value minus the first predetermined value, multiplied by a second predetermined value, when the absolute value of the first error signal is greater than the first predetermined value, and the first error signal is greater than the first predetermined value, and substantially indicative of the first error signal value plus the first predetermined value, multiplied by the second predetermined value, when the absolute value of the first error signal is greater than the first predetermined value, and the first error signal is not greater than the first predetermined value.
A third processing device is coupled with the second processing device to receive the second control signal and the first feedback signal. The third processing device transmits an output signal as a function of the second control signal and the first feedback signal. A fourth processing device is coupled with the third processing device to receive the output signal. The fourth processing device transmits the first feedback signal as a function of the output signal to the first processing device and the third processing device, the first feedback signal being substantially equal to the output signal delayed by a first predetermined duration of time.